1. Field of the Invention
The present invention relates to a process for the separation of propane and propylene by fractionation. More particularly the invention relates to an improvement in the operation of a propane-propylene splitter employing compressed overheads to heat reboiler bottoms to provide a portion of the energy used for the fractionation.
2. Related Art
The use of heat exchange (usually indirect) of one stream to heat another is widely practiced. For example in U.S. Pat. No. 2,619,814 (Kniel) the reboiler may be heated by compressed overheads, in U.S. Pat. No. 3,509,728 (Mercer) compressed overheads heated an evaporator and in U.S. Pat. No. 4,022,597 (Bacon) overheads and bottoms are used to heat incoming feed.
In the last several years propane-propylene splitters, i.e., fractionation units used to separate propylene from propane, have employed heat pumps to increase the pressure of the overhead vapors, thereby increasing the condensing temperature of the vapors, which are then heat exchanged with bottoms from the column in a reboiler to recover some of the heat and use it to drive the distillation.
U.S. Pat. No. 4,230,535 to Howard discloses that two close boiling chemicals may be separated by removing an overhead vapor stream, condensing a portion of the vapor, heating the vapor stream then compressing the heated vapors and supplying heat to a reboiler by condensing a portion of the compressed vapor stream.
U.S. Pat. No. 3,002,358 to Dierl discloses a propylene distillation wherein a portion of the overhead is compressed and used to heat two reboilers and a portion returned to the column as reflux.
In a C.sub.4 fractionation system disclosed in U.S. Pat. No. 4,559,108 to Ahlberg the overhead is split, a portion is compressed, used to heat the bottoms, then recombined with the uncompressed portion, cooled and collected for product recovery and/or reflux.
U.S. Pat. No. 4,277,268 to Spangler, Jr. discloses a fractionator split into a stripping section and a rectifying section wherein the overhead vapors from the rectifying section are heat exchanged with the bottoms of the stripping section.
J. Flores, et al., "Recompression Saves Energy," Hydrocarbon Processing, July 1984, pp 59-62, suggest that heat exchange of reflux with overhead will heat the overhead which cools the reflux but desirably assures that reflux will always be introduced at a temperature to flash and produce vapor to be recycled with overhead vapors. In practice it has been found that one of the bottlenecks in operating a C.sub.3 splitter has been the reflux, that is, when the reflux is returned to the column at an elevated temperature, more of it vaporizes, thereby creating excessive vapor that has to be handled in the column thus reducing its efficiency. Hence, the Flores suggestion would exacerbate the problem to which the present invention is directed.
Many propylene splitters are now operated with a heat pump employed to compress a portion of the overheads to thereby raise the condensing temperature of the overheads which are then heat exchanged in a reboiler with a portion of the bottoms to provide heat for the boil up in the column. The contact in the reboiler condenses the overheads which are accumulated, with a portion being yielded to propylene product and a portion being returned as reflux to the splitter (fractionating column). Reflux is returned to the column as a mixture of vapor and liquid, usually on to or above the first tray. Even though the temperature gradient along the propane-propylene splitter is generally less than 35.degree. F., the pressure of the overheads needs to be 150-180 psig or more, to provide a sufficient temperature differential in the reboiler (heat exchanger) to maintain the temperature in the bottom of the column for boil up and to drive the fractionation.
In the prior operation the temperature of the reflux is such that around 19% of the reflux was vaporized on the top tray, which increased the vapor from the top tray by that amount and increased the flow through the compressor, correspondingly.
In the present invention it was found that reducing the temperature of the reflux to a temperature no lower than temperature on the top tray and no higher than about 15.degree. F. greater than the temperature on the top tray, reduces the amount of reflux vaporized and allows the column to be operated at higher throughputs, hence increasing the columns' efficiency. Reflux returned to the tower is less likely to vaporize if the temperature is closer to the temperature on the top tray but no lower than temperature on the top tray.